Thank you for a such a detailed answer.
Actually I meant a bit different. Usually particular validation constraints belong to specific fields. I mean for some attributes it’s ok to use svg, in others only raster are required, so validation better be related to specific model attribute instead of all files upload.
What i meant was this type of in-form attribute specific error message 
Anyway I dug up some code in strapi-plugin-content-manager and in-form error messages like that generated on back-end seem impossible right now.
Actually the error message about uniqueness validation fail (back-end generated) looks like this 
Kind of not really readable to end user 